A factory fire in eastern China raged out of control unnoticed for hours Monday, after local residents were unable to distinguish the smoke from the dense smog that has filled the region.

The official Xinhua news agency says the furniture factory in Zheijiang province burned for nearly three hours before residents noticed the blaze. It took 10 hours to extinguish the flames, which destroyed a large amount of furniture.

Much of China's east coast has experienced its worst air pollution in recent memory over the past several days. In Beijing, a thick blanket of brown smog reduced visibility to just 200 meters, with the tops of many of the city's massive skyscrapers disappearing in the haze.

At its worst, official air quality readings in the capital reached nearly 40 times the World Health Organization's safe limit. Many hospitals were filled with patients complaining of heart or respiratory sicknesses.

The crisis has prompted an unprecedented amount of criticism of the government in China's state-controlled media, many of which published front-page commentaries on the issue. Government officials traditionally have downplayed the severity of the problem.

Facing increasing public outrage, China has taken steps in recent years to become more transparent about air pollution levels. It now publishes hourly updates online for more than 70 cities.

Conditions on Tuesday had improved slightly in the capital, as a cold front moved into the area. But officials warn that heavy smog will persist through Wednesday.